(a) Field of the Invention
The present invention relates to an LCD (liquid crystal display) device and, more particularly, to an LCD device of a transflective or reflective type having two polarizing films.
(b) Description of the Related Art
An LCD device generally includes a light source, and a light valve for controlling transmission or interception of the light emitted by the light source, on a pixel-by-pixel basis. An active-matrix-drive LCD has a light valve which controls an array of TFTs (thin film transistors) or MIM (metal-insulator-metal) diodes in the pixels and achieves a high-density and high-quality image display for the LCD device.
LCD devices are generally categorized in two types: a transmissive LCD device having therein a backlight unit as a light source; and a reflective LCD device having therein a reflecting film which reflects external light incident onto the LCD device and thus functions as a light source. The reflective LCD device has the advantages of lower power dissipation, and smaller thickness and weight compared to the transmissive LCD device, due to disuse of backlight unit in the reflective LCD device, and is mainly used in a portable terminal which requires smaller size and weight. On the other hand, the transmissive LCD device is superior to the reflective LCD device in that the transmissive LCD device can be well observed in a dark environment.
There is another type of the LCD device, known as a transflective LCD device, which has the advantages of both the reflective and transmissive LCD devices. Patent Publication JP-A-2003-156756 describes a transflective LCD device including a transmission area or transparent area in a part of pixel array formed on a rear substrate, and a reflection area in another part of the pixel array. In the reflection area, a pixel electrode has a function of polarizing the reflected light, whereby only a single polarizing film is disposed in the reflection area. Thus, this type of the LCD device is also referred to as a single-polarizing-film LCD device.
The LCD device described in the patent publication can be used as a reflective LCD device in a bright environment while turning off the backlight unit to operate the LCD device in a reflective mode, and as a transmissive LCD device in a dark environment while turning on the backlight unit to operate the LCD device in a transmissive mode. Thus, the transflective LCD device has the advantage of the reflective LCD device as to the lower power dissipation in the bright environment as well as the advantage of the transmissive LCD device as to the higher image-display capability in the dark environment.
It is noted that incident light passing through the reflection area has a path length different from the path length of incident light passing through the transmission area, because the former incident light passes twice through the liquid crystal (LC) layer whereas the latter incident light passes only once through the LC layer. This difference in the path length differentiates the retardation of the LCD device between the reflection area and the transmission area, and thus makes it difficult to optimize the intensity of the output light. Another problem is also involved in that the contrast ratio is reduced, or a specific color is observed in the monochrome image, during operation either in the reflective mode or in the transmissive mode.
For solving the above problems in the transflective LCD device, there are two known countermeasures; one which provides a large step difference between the reflection area and the transmission area; and another which optimizes the reflectance of an optical compensation layer. However, since those problems are essential to the LCD device having a single polarizing film, the known countermeasures do not provide a complete solution, and can only reduce the influence by the problems while increasing the costs of the LCD device.
In addition to the single-polarizing-film LCD device as described above, there is another type of transflective LCD device, referred to herein as a double-polarizing-film LCD device, wherein a polarizing film is provided on each of the front side of the front substrate and the rear side of the rear substrate, and wherein a reflection film (reflection plate) is provided on the rear side of the polarizing film disposed on the rear side of the rear substrate. This type of the transflective LCD device can be designed to optimize the intensity of the output light both in the transmissive mode and in the reflective mode, and prevent the reduced contrast ratio and coloring in the monochrome image.
In the double-polarizing-film LCD device, however, there are a parallax problem wherein the image is duplicated on the screen as viewed from the slanted direction in the reflective mode, a mixed-color problem wherein an undesirable color appears on the screen, and a reduced-luminance problem wherein some pixels have reduced luminances. These problems in the transflective LCD device are common to the reflective LCD device.